Thanks for your quick reply much appreciated, yer was thinking about the weight myself but just was not to sure about it

The difference between the lower side and the higher side is around a foot, so i assume what your saying is dig out the higher side to level out the site were i want the pool to sit?

Gradient ? sorry not to sure what you mean, i was thinking of building a frame using railway sleepers which are around a foot in width and 6 foot long, and then filling this smooth pebbles to raise it to one height

Compaction of pebbles/soil/whatever you use under the weight of water means you will have enormous difficulty getting the floor of the pool to remain level. Over time you will get areas dropping, making at best for a lumpy bottom of the pool and at worst a risk of the metal pool frame collapsing.

So after all that my advice is to dig out the higher side to level out the site. Even after that I highly recommend you hire a compactor to compact the soil on the level site prior to assembling the pool.

And if you're lucky, depending on your soil type you may not need a retaining wall on the bank of soil that remains on the "high" side of the pool.

BTW, gradient is a measure of the change in height over the distance. Yours appears to be 1 in 15.